A yekke is a Jew of German-speaking origin. The term carries the connotation that German Jews are notable for attention to detail and punctuality.
There are a number of theories regarding the etymology of the word. The best known is that it originates from the cultural differences in dress that developed between the more westernized Western European Jews who wore shorter "jackets" ("yekke", cf. German Jacke with an initial y-sound instead of the English j-sound) from the traditional longer coats while the outer clothing worn by the Eastern European Jews was typically longer.
Another theory is that the word derives from "Yekkef", the Western European pronunciation of the name "Jacob" or "Jack", which differs from the Eastern European pronunciation, "Yankef" or "Yankev" (also see German Jäckel, pronounced "yekkel", the German diminutive form of Jakob).
Furthermore, since Yiddish is derived from Middle High German, German sources occasionally note that Yiddish yekk(e) is largely homophone with jeck(e), the Rhenish dialectal form of Standard German Geck of uncertain origin but in contexts relating German jeck(e) to Yiddish yekk(e) attributed to patriarch Jacob in the Bible as does above-mentioned Yekkef theory.
Rhenish Jeck(e) as a noun denotes a "fool", "jester", or "crazy person" (both in a general sense, and in particular in association with the Rhenish carnival where the term describes a traditional folkloric stage figure akin to the Italian harlequin as well as merely a reveler celebrating the carnival season in a "foolish" manner due to ebriety) and as an adjective means "crazy", "mad", "insane", whereas the dated High German word Geck originally meant a "fool" as well but later transformed to particularly signify a "dandy" or "fop".
However, the broader usage of the Rhenish noun Jeck(e) in the Rhineland area has given the term a general meaning resembling "person", "(mere) mortal", or "humble sinner" with a particularly (self-)ironic connotation, such as in the common saying Jeder Jeck is anders (lit. "Every fool is different", with an idiomatic meaning of "Different strokes for different folks" or "Live and let live"). From there, a transformation into an ethnonym such as for Jewish people would be akin to the fate of similar terms such as Germanic man (word) and *þiuda-, and the equivalent words for "person" or "human being" in many other languages around the world.